An optical receiver used in a PON (passive optical network) system or the like receives optical signals in burst form sent from a plurality of subscriber terminals with distances from the respective subscriber terminals being different. In this case, amplitudes of the received baseband digital signals in burst form greatly change over time, and an amplitude ratio between a large-amplitude burst signal and a small-amplitude burst signal may reach even 1000 to 1. On the other hand, with respect to an input current signal of a receiver circuit in the optical receiver of this type, an offset current of a given level is generated due to an influence caused by a poor extinction ratio of an optical signal output by a transmitter, reflected return light, a dark current generated in a light receiving element of the receiver, and the like. At the same time, among carriers generated within the light receiving element in response to incidence of light, a carrier generated in a location within the light receiving element with low field intensity spreads slowly and gradually. Thus, an offset current that changes with a time constant larger than a clock period of the signal is generated.
The offset current which changes with the large time constant as described above has a frequency characteristic as shown at (a) of FIG. 22. That is, the light receiving element exhibits a high-frequency attenuation characteristic that has a shoulder at a frequency of several kHz to several hundred kHz. When optical waveforms of a large-amplitude burst signal (packet A) and a small-amplitude burst signal (packet B) that follows the large-amplitude burst signal as shown at (b) of FIG. 22 are input to the light receiving element having such a characteristic, a “0” level of an output signal of the light receiving element rises due to a low frequency response to the packet A, as shown at (c) of FIG. 22. Then, at a leading edge portion of the packet B that follows the packet A, the level is raised, and then gradually falls to the original “0” level. Such a phenomenon is herein referred to as a “tail”.
In the optical receiver used in the PON system or the like, in which the offset current as described above is present, it is important to receive the small-amplitude burst signal to be superimposed on the tail that appears immediately after the large-amplitude burst signal, with no errors. The burst signal is a unipolar signal. Thus, just by performing discrimination between the burst signals using a given threshold value, the leading edge portion of the small-amplitude burst signal immediately after the large-amplitude burst signal may be embedded in the tail, so that the leading edge portion of the small-amplitude burst signal will not be able to be received. Alternatively, when the threshold value is set so that the leading edge portion of the small-amplitude burst can be received, an end portion of the small-amplitude burst will not be able to be received. That is, when a current signal having such as offset is amplified, a duty ratio of an output waveform may remarkably fluctuate. Thus, accurate discrimination of a logic “0” level or a logic “1” level may become difficult.
Among duty ratio fluctuations caused by these offset currents, in regard to a duty ratio fluctuation caused by the offset current of the given level, various types of conventional techniques which suppress the duty ratio fluctuation caused by the offset current of the given level have been proposed. Patent Document 1, for example, describes an optical receiver circuit in which an electrical signal received from a light receiving element is converted to positive-phase and negative-phase output signals by a pre-amplifier, peaks of the signals are held, and added through a feedforward connection. In this optical receiver circuit, degradation in the duty ratio of an output waveform due to an input signal and an offset of the pre-amplifier is not generated. Thus, even when a received signal level is small, a margin for data discrimination is not degraded.
However, in the optical receiver circuit described in Patent Document 1, the offset that transitionally varies according to an elapsed time cannot be suppressed. The duty ratio of the output waveform will therefore remarkably vary. An influence of the offset that transitionally varies according to an elapsed time on the duty ratio variation of the output waveform appears noticeably especially when a large reception dynamic range as in the PON system is required.
Then, Patent Document 2 discloses an offset control circuit and an optical receiver that uses the offset control circuit, which can eliminate the offset that transitionally varies according to an elapsed time. With the offset control circuit and the optical receiver that uses the offset control circuit, even when such a large reception dynamic range is required, or even when optical signals in burst form, of which level differences are greatly different, are received, an output waveform free of fluctuations in duty ratio can be obtained.
FIG. 23 is a block diagram showing a configuration of the optical receiver described in Patent Document 2. This optical receiver is constituted from an optical receiving element 100 that converts an optical signal to a current signal IPD, a pre-amplifier circuit 120 that converts the current signal IPD output by the light receiving element 100 to a voltage signal, and amplifies the voltage signal, thereby outputting a positive-phase input signal VINP and a negative-phase input signal VINN, which are differential voltage signals, an offset control circuit 130 that cancels an offset that transitionally changes according to an elapsed time, and a discrimination level control circuit 140 that eliminates an offset of a certain level that does not change temporally and also sets a threshold value for performing discrimination between the logic “0” and “1” levels.
The offset control circuit 130 includes peak value hold circuits 132 and 131 that hold peak values of the positive-phase input signal VINP and the negative-phase input signal VINN output by the pre-amplifier circuit 120, respectively, doubling circuits 135 and 136 that double an output signal PDIN of the peak value hold circuit 131 and an output signal PDIP of the peak value hold circuit 132, respectively, an adder circuit 137 that adds an output signal of the doubling circuit 135 and the positive-phase input signal VINP, an adder circuit 138 that adds an output signal of the doubling circuit 136 and the negative-phase input signal VINN, and a differential amplifier circuit 139 that amplifies outputs of the adder circuits 137 and 138.
The discrimination level control circuit 140 is the circuit disclosed in Patent Document 1 or the like. The discrimination level control circuit 140 includes peak value hold circuits 142 and 141 that hold peak values of a positive-phase signal VO1P and a negative-phase signal VO1N output by the differential amplifier circuit 139, respectively, an adder circuit 143 that adds an output signal PDD2N of the peak value hold circuit 141 and the positive-phase signal VO1P, an adder circuit 144 that adds an output signal PDD2P of the peak value hold circuit 142 and the negative-phase signal VO1N, and a differential amplifier circuit 145 that amplifies an output signal AD2P of the adder circuit 143 and an output signal AD2N of the adder circuit 144. By comparing values of the output signals VO2P and VO2N of the differential amplifier circuit 145, a binary (values of “1” and “0”) digital signal COMPOUT is obtained.
Next, waveforms of respective portions of the optical receiver configured as described above will be described. FIGS. 24 and 25 are diagrams showing the waveforms of the respective portions of the optical receiver in FIG. 23. Referring to FIG. 24, the positive-phase input signal VINP and the negative-phase input signal VINN are shown. The current signal IPD with a tail is emitted from the light receiving element 100, and passed through the pre-amplifier circuit 120, thereby generating a differential voltage signal pair of the positive-phase input signal VINP and the negative-phase input signal VINN. Due to application of the tail, an envelope of the positive-phase input signal VINP monotonously decreases, while an envelope of the negative-phase input signal VINN monotonously increases. Accordingly, the output signal PD1N that indicates the peak value held in the peak value hold circuit 131 follows the tail, thereby following a signal amplitude peak value. On contrast therewith, the output signal PD1P that indicates the peak value held in the peak value hold circuit 132 does not reflect a signal amplitude peak value. The output signal AD1P is obtained by the addition by the adder 137 to the output signal PD1N through the doubling circuit 135. The output signal AD1N is obtained by the addition by the adder 138 to the output signal PD1P through the doubling circuit 136.
Further, as shown in FIG. 25, a difference voltage between the output signals VO1P and VO1N obtained by amplifying a difference voltage between the output signals AD1P and AD1N by the differential amplifier 139 becomes a unipolar signal without the tail but with the offset. By inputting the output signals VO1P and VO1N to the discrimination level control circuit 140, which is a unipolar to bipolar signal converter circuit, the output signals VO2P and VO2N of bipolar signals can be obtained as outputs of the discrimination level conversion circuit 140. By comparing these signals by a comparator, the binary digital signal COMPOUT that has a satisfactory duty ratio even when the light receiving element 100 emits a current with a tail can be obtained.
Patent Document 3 discloses a signal amplifying circuit that handles various transient responses that may occur at a leading edge of a burst cell and is also resistant to disturbances such as extraneous noise. This signal amplifier circuit includes a first level detection circuit that detects a direct current level of a positive-phase signal, a first adder circuit that adds a negative-phase signal to a detection output of the first level detection circuit, a second level detection circuit that detects a direct current level of the negative-phase signal, a second adder circuit that adds the positive-phase signal to a detection output of the second level detection circuit, and a differential amplifier circuit that differentially amplifies outputs of the first and second adder circuits. Then, each of the first and second level detection circuits includes a peak detection circuit that detects a maximum value of the positive-phase signal, a bottom detection circuit that detects a relative minimum value of the positive-phase signal based on a detection level of the peak detection circuit, and a voltage divider circuit that performs voltage division of detection outputs of the peak detection circuit and the bottom detection circuit.
Further, Patent Document 4 discloses as a related device an optical burst receiving apparatus that allows maintenance of a high transmission efficiency without causing reception inability resulting from a frequency response or deterioration of a code error rate.    [Patent Document 1] JP Patent No. 2656734    [Patent Document 2] JP Patent No. 3606143    [Patent Document 3] International Publication W001/048914A1    [Patent Document 4] JP Patent Kokai Publication No. JP-A-11-112439